This only happens in missions with a shop in the briefing sequence. When you decide to drop the lantern it doesn't happen.
In mission A matter of hours you can drop the lantern in the shop, but when the mission is started you still have it. But then the double light bug doesn't happen. If you don't drop it, the bug happens.
In Cleaning up the neigborhood there's no shop, but you do start with the lantern, so the bug doesn't happen.
I tested this by pressing the L key just before clicking on the "Press Attack to start mission" screen.